1. Field of the Invention
This invention is related to wellbore mills, wellbore milling processes, milling tools and whipstocks; and in one aspect to milling processes which employ a diverter or a whipstock. Various milling methods and systems are disclosed.
2. Description of Related Art
In wellbore operations, milling tools are used to cut out windows or pockets from a tubular, e.g. for directional drilling and sidetracking; and to remove materials downhole in a well bore, such as pipe, casing, casing liners, tubing, or jammed tools. Many wellbore milling tools have a plurality of cutting elements or "inserts" secured to milling blades and/or milling surfaces. Typically these inserts are fixed on the blades by brazing or welding. In certain prior art mills, holes are provided into which part of the insert is inserted and by which the insert is held in place prior to brazing or welding.
A variety of problems are associated with efforts to braze or weld inserts onto milling tools. Many types of welding require specific material pretreatments and specific heat treatments before, during, and after welding. When welding methods are used undesirable temperature-induced changes to a base metal or to an insert may result. These changes can be irreversible. Also with such methods inserts may be secured in consistently, i.e. adhesion may differ from insert to insert. Variations in thermal coefficients of expansion between materials e.g. between carbides and bonding materials, can result in undesirable cracks during cooling. Verification of adhesion quality is difficult. If brazing is employed, carbide selection may be limited, e.g. possibly coated carbides may not be usable. With certain welding methods inserts are not precisely located and are placed inconsistently.
The prior art discloses various types of milling or cutting tools provided for cutting or milling existing pipe or casing previously installed in a well. These tools have cutting blades or surfaces and are lowered into the well or casing and then rotated in a cutting operation. With certain tools, a suitable drilling fluid is pumped down a central bore of a tool for discharge beneath the cutting blades and an upward flow of the discharged fluid in the annulus outside the tool removes from the well cuttings or chips resulting from the cutting operation.
A section of existing casing can be removed from a well bore with a milling tool, e.g. to permit a sidetracking operation in directional drilling, or to provide a perforated production zone at a desired level. Also, milling tools are used for milling or reaming collapsed casing, for removing burrs or other imperfections from windows in the casing system, for placing whipstocks in directional drilling, or for aiding in correcting dented or mashed-in areas of casing or the like.
Certain prior art sidetracking methods use cutting tools of the type having cutting blades and use a deflector such as a whipstock to cause the tool to be moved laterally while it is being moved downwardly in the well during rotation of the tool to cut an elongated opening pocket, or window in the well casing.
Various prior art well sidetracking operations employ a whipstock and a variety of different milling tools used in a certain sequence. This sequence of operation requires a plurality of "trips" into the wellbore. For example, in certain multi-trip operations, a packer is set in a wellbore at a desired location. This packer acts as an anchor against which tools above it may be urged to activate different tool functions. The packer typically has a key or other orientation indicating member. The packer's orientation is checked by running a tool such as a gyroscope indicator into the wellbore. A whipstock-mill combination tool is then run into the wellbore by first properly orienting a stinger at the bottom of the tool with respect to a concave face of the tool's whipstock. Splined connections between a stinger and the tool body facilitate correct stinger orientation. A starting mill is secured at the top of the whipstock, e.g. with a setting stud and nut. The tool is then lowered into the wellbore so that the packer engages the stinger and the tool is oriented. Slips extend from the stinger and engage the side of the wellbore to prevent movement of the tool in the wellbore. Moving the tool then shears the setting stud, freeing the starting mill from the tool. Rotation of the string with the starting mill rotates the mill. The starting mill has a tapered portion which is slowly lowered to contact a pilot lug on the concave face of the whipstock. This forces the starting mill into the casing to mill off the pilot lug and cut an initial window in the casing. The starting mill is then removed from the wellbore. A window mill, e.g. on a flexible joint of drill pipe, is lowered into the wellbore and rotated to mill down from the initial window formed by the starting mill. Typically then a window mill with a watermelon mill mills all the way down the concave face of the whipstock forming a desired cut-out window in the casing. This may take multiple trips. Then, the used window mill is removed and a new window mill and string mill and a watermelon mill are run into the wellbore with a drill collar (for rigidity) on top of the watermelon mill to lengthen and straighten out the window and smooth out the window-casing-open-hole transition area. The tool is then removed from the wellbore.
There has long been a need for an efficient and effective milling method which is not dependent on an insert brazing or welding method. There has long been a need for wellbore milling with an optimum density of cutting inserts. There has long been a need, recognized by the present inventors, for a mill with inserts installed without brazing or welding. There has long been a need for such a mill with precisely placed inserts. There has long been a need for such a mill that is effective in milling relatively hard material that is difficult to machine, e.g. but not limited to, high chrome casing.